Facsimile safety device



March 6, 1951 F. A. HESTER ET AL FACSIMILE SAFETY DEVICE Filed Aug. 10,1946 INVENTORS FRANK A. HESTER 8 JOHN WSMJH AT TOR/YE Y Patented Mar. 6,1951 FACSIMILE SAFETY DEVICE Frank A. Hester, New York, and John w.Smith,

Long Island City,

N. Y., assignors to Faximile,

Inc., New York, N. Y., a corporation of Delaware Application August 10,1946, Serial N 0. 689,791

2 Claims.

The present invention relates to facsimile recorders and in particularto safety devices for electrolytic facsimile recorders.

One object of the present invention is to provide a new and improvedmeans for controlling the current to the marking electrode of anelectrolytic recording device.

Another object of the present invention is to provide a new and novelmeans for protecting the output tubes in a facsimile recorder when theload is suddenly removed.

Still another object of the present invention is to provide an improvedmethod and means for controlling the marking current in an electrolyticrecorder in order to accommodate a wide range of impedance values due tovarying moisture content of the recording paper.

A further object is to prevent burning of an electrolytic recordingpaper which may be excessively dry.

In practicing the art of facsimilea subject copy comprising pictures,graphic material, or textual matter is scanned and impulsescorresponding to density variations are transmitted to a distant pointwhere they are reproduced on a suitable carrier. This carrier may be aphotographic film, a paper tape or sheet or other suitable medium. Onecarrier which has come into considerable use consists of a paper sheetor tape impregnated with an electrolytic solution which may be marked bya varying electric current. Marking may be accomplished by passing acurrent between a rotating helical electrode and a flexible linearelectrode pressing upon opposite sides of the paper. At any given speedand with constant electrode intersecting areas the density of the markproduced is substantially proportional to the current passing throughthe paper. Hence one of the requirements of such a system is the veryclose control of the recording current. It has been found, however, thatvarying paper stocks, varying paper thicknesses, and variations inmoisture content in the paper may give rise to considerable impedancevariations between the recording electrodes.

According to the present invention, greatly improved fidelity in therecording in the presence of paper impedance variations is accomplishedby providing marking current which is substantially independent of thesevariations.

This is accomplished by feeding the marking electrodes by means of adevice which determines the marking current independent of the paperimpedance. A preferred feeding device is a socalled constant currentvacuum tube. Tetrodes,

pentodes or beam tetrodes may be used. Triodes with a large amount ofcurrent feed-back may also be used. A pair of tetrodes connected inparallel is shown in the drawing to illustrate the operation of theinvention.

In the drawing Figure 1 shows a circuit of a preferred embodiment of thepresent invention.

Figure 2 shows curves illustrating the operation of Figure 1.

Figure 1 shows a coupling unit I receiving facsimile signals to berecorded, over leads 2 and 3. The output of coupling unit I consistingof rectified facsimile signals is fed through the circuit to bedescribed over leads 4 and 5. Two tetrode tubes 6 and 1 are utilized inthe embodiment shown. Tetrode tube 6 includes a cathode l2 heated by aconventional heater not shown, a control grid I3, a screen grid Hand aplate l5. The tetrode tube 1 includes cathode l6 heated by aconventional heater means not shown, control grid l1, screen grid [8,and plate IS. The rectified facsimile signals from coupling unit Icarried by lead 4 are applied to grid [3 and grid H. A

suppressor resistor 20 for suppressing undesired spurious oscillationsmay be included as shown in the lead to grid I1. Plate and. screenvoltages for tubes 6 and I are supplied from a conventional directcurrent source such as battery 23. Cathodes l2 and I6 are connected tothe negative side of battery 23 and the return circuit from couplingunit I through lead 5 is returned to the negative end of battery 23through the bias battery 22. Bias battery 22 is suflicient to operatetubes 6 and l at substantially cut-olT in the absence of facsimilesignals from coupling unit I.

.The positive end of battery 23 is connected to screen grid I8 throughthe circuit consisting of relay coil 29 shunted by variable resistor 28in series with resistor 21 shunted by relay contacts 30 and 3| and maypass through a spurious oscillation suppressor resistor- 2|. The screenvoltage for screen I4 is similarly supplied except that a secondsuppressor resistor 26 may be used as shown. The voltage from battery 23reaches plates l5 and I9 through a circuit including the linear printerelectrode ID, the electrolytic recording paper I I and the helix wire 8.Suppressor resistor 24 may be included in series with plate l5 andresistor 25 in series with plate [9 to suppress undesired spuriousoscillation. Variable resistor 32 may also be included in series withthe plate circuits to enable adjustment of the overall series resistancefrom the battery to the plates.

The facsimile signals to be recorded appear at the output of couplingunit I as positive pulses of electric current and these serve to unblocktubes 6 and I causing plate current to flow through the plate circuitsand thus between linear electrode l and helical electrode 8 through theelectrolytic recording paper II. Helical electrode 8 is rotated as it isattached to the surface of rotating drum 9. The intersection betweenlinear electrode [0 and rotating helical electrode 8 forms the scanningspot which moves across the paper. The electrolytic recording sheet IIis advanced at a constant rate of speed by conventional means not shownwhich completes the scanning operation to cover the entire sheet fromtop to bottom. The varying currents passed through sheet H cause itsoptical density to change as a function of the current and thus producemarks on the paper producing the facsimile copy. In order that thefacsimile record shall be a true reproduction of the original, it isnecessary that the current passing through the electrolytic paper I i bea predetermined function of the signals received by tubes 6 and I fromcoupling unit I. If the rectified signals from coupling unit I arepassed directly through electrolytic recording paper I l the currentwil1 vary considerably due to variations in the impedance of the paper.This variation of the paper impedance may be produced by a number ofcauses. The paper stock, the thickness or the sheet, the strength of theelectrolyte in the paper, and the amount of moisture in the paper allaffect the impedance of the recording sheet.

Figure 2 shows several curves illustrating the operation of Figure 1.Curve A shows the plate current of tubes 6 and I plotted against platevoltage. It has been found that the plate voltage may be increased frompoint D to point F with a very slight change in plate current. Since thevariations in the impedance of the recording paper is equivalent to avariation in plate voltage the characteristics of tetrodes 6 and Ioperate to maintain constant marking current for a constant grid voltageover a wide range of paper conditions.

It has been found that not only is the fidelity of the recordingimproved but that a new and unexpected result has been achieved. Whilethe electrolytic recording paper is used in a moist condition undercertain circumstances it may become quite dry as for instance uponstanding idle for some time. If a heavy current is passed throughsubstantially dry paper it may char or even catch on fire. The presentcontrol system has eliminated this trouble entirely since the currentcannot rise to a dangerous point.

Thus the system of the present invention provides means for improvedfidelity in facsmile recording by determining the recordingsubstantially independently of the paper condition or characteristics.It provides, in addition, the unexpected result of preventing burning ofthe paper. It also provides for lifting the linear electrode from thepaper by automatically modifying the control device to prevent damage toit under the extreme condition of infinite impedance. When the linearelectrode is lifted, the plate current drops to zero and the screencurrent rises as shown in Figure 2. Relay 29 is adjusted to open at somepredetermined value of screen current such as C in Figure 2, thusprotecting the tubes from damage. When contacts 30 and 3| open thescreen current limiting resistor 21 is placed in the circuit to providethis protection.

While a single embodiment of the present invention has been shown anddescribed, many modifications will be apparent to those skilled in theart within the spirit and scope of the invention as set forth in theappended claims.

What is claimed is:

1. In a device for electrolytic facsmile recording the combination of atleast one thermionic vacuum tube exhibiting constant-currentcharacteristics over at least a predetermined portion of its operatingcharacteristics, including at least a cathode, control grid, screen andanode, a source of direct current, and a pair of recording electrodes,including an electrolytic record ing sheet there-between all connectedin series between the cathode and plate or" said tube, a source of biasand a source of facsimile signals connected in series between thecathode and the control grid of said tube, and protective meansconnected between said source of current and the screen grid of saidtube including a relay having a pair of contacts and a resistorconnected across said contacts adjusted to open said contacts at apredetermined screen current level for limiting the current to saidscreen grid when said recording electrodes are not in contact with saidelectrolytic paper.

2. A device as set forth in claim 1 and including at least a secondconstant current tube connected in parallel with said first tube.

FRANK A. HESTEB. JOHN W. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,315,362 Wise Mar. 30, 19432,397,559 O'Brien Apr. 2, 1946

